Some other/old servers had specific user modes in a channel that would ban "just them" The one i remember is +q from chatspace (this is going back years).
The answer has been giving up above, use shun or the +b ban. I would recommend using the +b ban as you won't need to be a netadmin, the user will be aware that they are "quiet banned" and you won't affect them on other channels.
I know mIRC scripting, but it's been a while, you'll probably want to change the script to something much simpler...
The . instead of the / causes mirc to hide the command from you (so you don't see your client typing in /mode .....). You will still see the results of the command.
The '4' may need to change as it determins what mask is banned, check the mIRC scripting documentation to find the one that suits you best. Aslo, if you get creative, you can use Unreal's extended bans (~q for example) to get the exact results.
As for the Onotice, I don't know about you, but if an AOP was spamming me like that, they would be loosing their AOP pretty fast, i would recommend tossing the /onotice line completly.
Never argue with an idiot. They will bring you down to their level, then beat you with experience.